The number of floors on a skyscraper and the time it takes to get to the higher floors by the elevator.
The relationship is proportional because the higher the elevator goes up, the longer it will take to get there. For example:
Floor 1 takes 25 seconds
Floor 2 takes 50 seconds
Floor 3 takes 1 minute 15 seconds
Floor 4 takes 1 minute 40 seconds
and so on.
